1. Field of the Invention
This invention relates to a charge transfer device in which charge carriers are transferred in the vicinity of the surface of a semiconductor by potential wells accompanying an acoustic wave propagating in a piezo-electric material. This invention more particularly relates to a charge transfer device of monolithic type which can be used as an image information pick-up device.
2. Description of the Prior Art
There have heretofore been known various devices for transferring information. For instance, as the devices utilizing transfer of magnetic polarity are known so-called magnetic bubble devices or devices using a magnetic core with a hole in which magnetic polarity stored therein is transferred. As the devices utilizing transfer of electric charges are known, for instance, so-called BBD (bucket brigade device) in which circuits consisting of a MOSFET (metal oxide semiconductor field effect transistor) and a capacitor are integrated, and so-called CCD (charge coupled device) in which minority charge carriers are transferred along the surface of a semiconductor by use of moving potential wells created by successively applying a voltage on a series of electrodes provided on an insulating oxidized film on the semiconductor. The CCD has attracted special interest recently.
However, these devices have various drawbacks as follows. For instance, the magnetic bubble device has a long access time as long as 1 msec, and BBD has a great loss of charge carriers in transfer thereof (hereinafter referred to as "transfer loss"). The CCD is disadvantageous in that the transfer speed is limited by the diffusion speed of the charges since the potential under the transfer electrodes is flat and a part of the charge carriers move at the diffusion speed thereof. Further, CCD requires a series of electrodes for transfer and lead wires for applying voltage successively to the electrodes, and accordingly the structure is complex and hard to manufacture.
Beside the above mentioned devices, devices in which charge carriers are transferred in the vicinity of the surface of a semiconductor by use of an acoustic wave which propagates within a piezo-electric material are known in the art, which are called SAW-CTD (surface acoustic wave charge transfer device). These SAW-CTD are also disadvantageous in practical use and accordingly are not widely put into practical use. For instance, as shown in U.S. Pat. No. 3,858,232, the SAW-CTD provided with a series of metal electrodes between a semiconductor and a piezo-electric material is disadvantageous in that the potential has no gradient under the electrodes and accordingly the transfer speed is limited by the diffusion of the charge carriers per se. Further, in said U.S. patent there is shown a device in which a piezo-electric material is in direct contact with a semiconductor. Such a device has a defect in that there occurs injection or release of charge carriers between the semiconductor and the piezo-electric material and the injected charge carriers get into traps within the piezo-electric material, which results in lowering of the signal-to-noise ratio and results in hysteresis in the electric characteristics of the device. Further, another defect of the SAW-CTD is in that there is no channel stop for preventing displacement of charge carriers from charge transfer channels and accordingly a part of the charge carriers drop out of the transfer channels which results in transfer loss. Further, since there is no channel stop around the transfer channels, the transfer channels cannot be designed into non-linear form and accordingly the SAW-CTD must have an elongated shape when it is used as a delay line. Further, in Japanese Patent Publication No. 52(1977)-36677 there is disclosed a SAW-CTD in which an insulating layer is provided between a piezo-electric material and a semiconductor. This device also does not have a channel stop and accordingly has a great transfer loss and therefore is not put into practical use. In Ultrasonic Symposium Proceedings IEEE, Cat. #76, p.201-204, there is disclosed a SAW-CTD in which an insulating layer is provided on a semiconductor and a piezo-electric material is provided thereon and further thereon are disposed electrodes. This device, however, also does not have a channel stop and accordingly the transfer loss is too great. In Applied Physics Letter Vol. 29, No. 2, page 82, there is disclosed a SAW-CTD which has an air gap between a piezo-electric material and a semiconductor. This device is disadvantageous in that the electric characteristics such as electric potential are not uniform because of ununiform distribution of depletions and because the exposed surface at the air gap is liable to be electrically affected by stains or the like deposited thereon, and further in that the device is apt to be deformed because of the air gap and the output is unstable due to such defects.
As mentioned above, the conventional CCD's are impractical because of their limited transfer speed. In the conventional CCD's, the transfer frequency cannot be raised with a small transfer loss accordingly. The conventional SAW-CTD's are also impractical because of their great transfer loss and unstable electric characteristics. In addition, in the conventional SAW-CTD it is difficult to control the time of injection and amount of injection when charge carriers are injected since there is no injection gate.
Further, in case of the conventional SAW-CTD, it is impossible to store the charge carriers since the transfer of the charge carriers are caused by the potential wells moving accompanying a propagating acoustic wave. On the other hand, in the CCD the charge carriers can be stored under the electrodes when there is no movement of the potential since the transfer of the charge carriers are caused by the movement of potential between electrodes.
Furthermore, in any of the above-mentioned conventional devices, it is impossible to read out the stored information stored separately in two dimensional sense at a desired time interval.
In the SAW-CTD, the charge carriers are detected by means of a diode or the like. When the charge carriers transferred with potential wells propagating accompanying an acoustic wave are detected, the acoustic wave passing by the detection diode is detected in the form of piezo-electric potential since there is a piezo-electric material on the diode. Therefore, even where there is no signal or charge carriers trapped by the acoustic wave, the piezo-electric potential accompanying the acoustic wave is detected by the detection diode, which becomes a noise. Particularly when the amount of the charge carriers transferred is small, the noise based on the detected piezo-electric potential becomes relatively large.
Further, in the SAW-CTD used as an image pick-up device, there is a problem of blooming. That is, when the amount of the incident light is locally large, the charges in the potential well at the position of the large amount of incident light overflows into the adjacent potential wells, which results in blur of the image obtained through the device.